warlock tainted - best merc

I’m running a HC warlock , pure Tainted summoner. Still building up the Blood Oath and Engorge to support a bit physical damage.
I was wondering what are the best/ safe options for my merc? ( to mention I manage to evaporate 2 previous +85 HC warlock Abyss and Bind Demon ) but I don’t give up - I want this one to get to 99
I even run with CoH now on myself. Would a merc 5 be a better option than merc2, with more auras for Tainted? Should I prebuff and then run Infinity as my main? any ias on me doesn’t affect the Tainted so should in focus in fcr?
Until now, using Death Mark to place the Tainted was no risk at all - but getting harder with T5 heralds and too many demons around
Ps. Was reading already the existing posts about binding demons, prebuffing, skillets etc, but merc question is still open. Thx!

On hc i would honestly skip heralds, as there is literally no point to bother with such high risk. Since last patch i farmed from 91 to 94, mostly in tzs. There 100% was 100+ t5 harolds, which nets me 0 sunders charm. I also always clears act bosses, and got 2 magic, one phys, cold and lightning in the same time frame. With like zero risks at all.